I've used Monster XP for powering some mains in a bedroom setup in the past. I have since re-run complete wiring for the room as needed using standard Philips speaker wire (same gauge as Monster XP) from Home Depot as the Monster wiring was temporary. Oddly enough, I have noticed absolutely nothing different about the performance of the wiring change, though I really never expected there to be any difference.
I'm sure I could convince myself there is a difference, but honestly, I'd be kidding myself if I did. I'm not saying it couldn't happen, but if both sets of wire were built to the same specs, neither were faulty and they were both relatively similar in run length, you'd have to have dog hearing to notice a difference... -TD